Newswise — WASHINGTON — A recent study, published in the AGU journal Water Resources Research, has revealed that climate change has had a profound impact on the Colorado River Basin between the years 2000 and 2021. The study shows that over this period, more than 40 trillion liters (10 trillion gallons) of water were lost due to climate change effects, which is roughly equivalent to the entire storage capacity of Lake Mead.
The research indicates that without the influence of climate change, the drought experienced in the basin would not have led to reservoir levels reaching the critical point in 2021, necessitating the first-ever federally declared water shortage.
The study's lead author, Benjamin Bass from the University of California-Los Angeles, who specializes in hydrological modeling, expressed surprise at the basin's sensitivity to warming compared to other major basins in the western United States. Particularly, the study highlighted the high sensitivity in the crucial snowpack regions, which occupy a relatively small area in the basin.
The significance of the findings cannot be overlooked, as the study serves as a wake-up call to the severe impact of climate change on the availability of water in the Colorado River Basin. The removal of such a vast amount of water from the basin during the recent megadrought, equivalent to the volume of Lake Mead itself, emphasizes the urgent need to address the consequences of climate change that we are witnessing in the present day.
The Colorado River Basin, spanning about 647,500 square kilometers (250,000 square miles) and encompassing seven states in the U.S. West, plays a crucial role in supplying water to approximately 40 million people and supporting agriculture and natural ecosystems. However, the region has been facing a severe drought since around 2000, marking the driest period in the past 1,200 years. This prolonged drought has led to reduced river flow and diminished reservoir levels, raising concerns about water scarcity amid ongoing climate change.
While previous assessments of water resources in the area focused solely on the impact of climate processes, they neglected to consider the intricate responses of plants to higher atmospheric carbon dioxide levels. In contrast, the new study represents a significant improvement in hydrologic modeling for this region by incorporating the influence of changes in vegetation resulting from increased carbon dioxide levels. It stands as the first study to quantify the effect of anthropogenic climate change on traditional snowpack regions in the Colorado River Basin, revealing how it has contributed to the drying out of these areas and rapid reductions in the runoff that nourishes the Colorado River.
To investigate the changes in the basin's hydrology from 1880 to 2021, the researchers employed a comprehensive land surface model capable of analyzing water dynamics, vegetation changes, and vegetation responses to rising atmospheric carbon dioxide levels. The study utilized standard datasets to represent atmospheric conditions, incorporated streamflow gauges and other records to capture regional hydrological patterns, and integrated ground and satellite data to document actual transformations in vegetation over time.
Bass stated that by incorporating all these diverse types of data, their analysis covered all the significant contributors to runoff's sensitivity to climate change.
The model's findings revealed that between 1880 and 2021, the temperature in the Colorado River Basin rose by approximately 1.5 degrees Celsius (2.7 degrees Fahrenheit) due to human-induced climate change. This temperature increase resulted in a 10.3% reduction in runoff under current conditions. However, if the effects of vegetation were not considered, the water loss under present-day conditions would have been closer to 13%. This highlights the crucial role of including vegetation processes in water modeling, as emphasized by the study's authors.
This decline in runoff, amounting to a 10.3% reduction, reached its peak during the historical drought period spanning from 2000 to 2021. Throughout this drought, the cumulative volume of lost runoff water was approximately equal to the total capacity of Lake Mead, the largest reservoir on the Colorado River.
The researchers also made a significant discovery, finding that the snow-covered regions of the basin, which are typically abundant with water during winter, are experiencing water loss at a rate twice as fast as the usually snowless areas. This transition is raising immediate concerns among water managers because these snowpack regions play a disproportionately large role in the basin's water supply. Although they constitute only about one-third of the entire basin, they contribute to about two-thirds of its total runoff.
The alarming rate of water loss in the snowpack regions indicates a shift toward a more arid climate in the Rocky Mountain West, suggesting that it is not merely a temporary drought situation but a more profound and long-term change, as explained by Bass.
The 1922 Colorado River Compact, which governs the allocation of river water among the seven western U.S. states that the Colorado River flows through, was formulated with the assumption that the regional climate would remain stable. However, Bass highlights that if the representatives of these states were drawing up the contract today, they would need to adjust the water allocation value of 15 million acre-feet per year (approximately 18.5 trillion liters or 4.6 million gallons) due to the impact of warming. The study underlines that, going forward, while there may still be some natural variability with periods of wet or dry conditions, the trend of decreasing runoff is likely to persist if greenhouse gas emissions are not curbed.
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